Changes in tissue oxygen tension, venous saturation, and Fick‐based assessments of cardiac output during hyperoxia

BACKGROUND Hyperoxemia (arterial oxygen tension >100 mm Hg) may occur in critically ill patients and have effects on mixed venous saturation (SvO2 ) and on Fick-based estimates of cardiac output (CO). We investigated the effect of hyperoxemia on SvO2 and on assessments of CO using the Fick equation. METHODS Yorkshire swine (n = 14) were anesthetized, intubated, and paralyzed for instrumentation. SvO2 (co-oximetry) and tissue oxygen tension (tPO2 , implantable electrodes) in brain and myocardium were measured during systematic manipulation of arterial oxygen tension (PaO2 ) using graded hyperoxia (fraction of inspired oxygen 0.21 → 0.8). Secondarily, oxygen- and carbon dioxide-based estimates of CO (FickO2 and FickCO2 , respectively) were compared with measurements from a flow probe placed on the aortic root. RESULTS Independent of changes in measured oxygen delivery, cerebral and myocardial tPO2 increased in proportion to PaO2 , as did SvO2 (P